Technical Field
The present disclosure relates generally to the field of downhole logging tools and, more particularly, to an excluder for a nuclear magnetic resonance logging tool.
Background Information
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the subject matter described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present disclosure. Accordingly, it should be understood that these statements are to be read in this light, not as admissions of prior art.
Logging tools have long been used in wellbores to make, for example, formation evaluation measurements that support inferring properties of the formations surrounding the wellbore and the fluids in the formations. Common logging tools include electromagnetic (resistivity) tools, nuclear tools, acoustic tools, and nuclear magnetic resonance (NMR) tools, though various other types of tools for evaluating formation properties are also available.
Some conventional NMR tools include an antenna that emits a radio frequency (RF) magnetic field from a first or “front” side of the antenna. One concern with such tools in oilfield applications is their performance in wellbores containing high-salinity, conductive fluid (e.g., mud). The conductive mud appears as an additional resistive load (Rmud) due to eddy currents being generated in the conductive mud when the magnetic field is applied. The power absorbed due to the eddy currents may be evaluated using the equation below:Pabs=∫J·Edvwhere Pabs represents the power absorbed, J represents the current induced in the mud, and E represents the electric field. The additional load due to the mud may be expressed by:
      P    abs    =            1      2        ⁢          RI      2      where R represents the resistive load, and I represents current. It has been observed that this loss may also cause an additional Johnson-Nyquist noise due to thermal fluctuations in the mud that are proportional to the square root of Rmud.
The antenna of the NMR tool is positioned within a hollow, cylindrical sleeve (referred to as an excluder). The majority of the power absorbed is by the mud that is positioned between the front side of the antenna and the wellbore wall. The excluder may be placed in contact with the wellbore wall to reduce the amount of mud between the antenna and the wellbore wall, and thus the amount of power lost. However, because the excluder has a smaller diameter than the wellbore wall, any contact between the excluder and the wellbore wall is along a linear (e.g., vertical) contact line. Thus, the distance between the excluder and the wellbore wall, and the amount of mud between the excluder and the wellbore wall, increases moving circumferentially away from this linear contact line.